1. Field of the Invention
The present invention relates to boundary acoustic wave devices which are used for resonators and band-pass filters. In particular, the present invention relates to a boundary acoustic wave device that includes an IDT electrode provided between a first medium and a second medium and utilizes boundary acoustic waves propagating along the boundary between the first medium and the second medium.
2. Description of the Related Art
Surface acoustic wave devices have been widely used as resonators and band-pass filters. Recently, instead of such surface acoustic wave devices, boundary acoustic wave devices that enable package size reduction have been attracting attention.
For example, WO2004/070946 discloses a boundary acoustic wave device having a configuration as shown in FIG. 9. A boundary acoustic wave device 101 includes a first medium 102 and a second medium 103 that are stacked together. The first medium 102 is made of a LiNbO3 substrate and the second medium 103 is made of SiO2. Additionally, an IDT 104 made of Au is disposed at the boundary between the first medium 102 and the second medium 103.
Since the IDT 104 is made of the metal having a high density and a low acoustic wave velocity, vibrational energy is concentrated at a portion at which the IDT 104 is disposed, that is, at the boundary between the first medium 102 and the second medium 103. As a result, a boundary acoustic wave is excited.
Unfortunately, the boundary acoustic wave device 101 described in WO2004/070946 has a problem in temperature properties, that is, the boundary acoustic wave device 101 has a relatively large absolute value of a temperature coefficient of group delay time TCD. This problem will be specifically described. As disclosed in WO2004/070946, the boundary acoustic wave device 101 has a configuration in which the first medium 102 is made of a 15° Y-cut X-propagation LiNbO3 substrate and has a thickness of 8λ, the second medium 103 is made of SiO2 and has a thickness of 8λ, and the IDT electrode 104 is made an Au film having a thickness of 0.05λ and an Al film having a thickness of 0.05λ that are stacked on the Au film, and the boundary acoustic wave device 101 has a duty of 0.5. The temperature coefficient of group delay time TCD of this configuration was calculated.
The calculation was conducted by extending the finite element method described in “Finite-Element Analysis of Periodically Perturbed Piezoelectric Waveguides” (The Institute of Electronics and Communication Engineers Transactions, Vol. J68-C, No1, 1985/1, pp. 21-27). Specifically, one strip is disposed within an interval of a half wavelength and acoustic wave velocities at the upper end of a stop band and at the lower end of the stop band were determined in the open-circuited strip and in the short-circuited strip. The acoustic wave velocity at the lower end in the open-circuited strip is represented by V01. The acoustic wave velocity at the upper end in the open-circuited strip is represented by V02. The acoustic wave velocity at the lower end in the short-circuited strip is represented by VS1. The acoustic wave velocity at the upper end in the short-circuited strip is represented by VS2. The vibration of boundary acoustic wave is propagated such that most of the vibrational energy is concentrated in the range from a position above the IDT by 1λ to a position below the IDT by 1λ. For this reason, an analysis region was defined as a region of 8λ in the vertical direction with the IDT electrode being the center of the region, that is, a region from a position above the IDT electrode by 4λ to a position below the IDT electrode by 4λ. The boundary conditions of the front surface and the back surface of the boundary acoustic wave device were elastically fixed.
Then, κ12/k0 representing the amount of reflection of boundary acoustic wave in the electrode fingers of the IDT electrode and an electromechanical coefficient K2 were determined by a method described in “Evaluation of Excitation Property of Surface Acoustic Wave Interdigital Transducer By Mode Coupling Theory” The Institute of Electronics, Information and Communication Engineers Research Report, MW90-62, 1990, pp. 69-74). Compared to the configuration described in this document, the configuration used herein exhibits a larger frequency dispersion in acoustic wave velocity. For this reason, κ12/k0 was determined in consideration of the influence of the frequency dispersion.
The temperature coefficient of group delay time TCD was calculated from phase velocities V15° C., V25° C., and V35° C. at the lower end of the stop band of the short-circuited strip respectively at 15° C., 25° C., and 35° C.
         Equation    ⁢                  ⁢    1    ⁢                              ⁢                          ⁢                            ⁢                  ⁢                                        TCD            =                                          α                s                            -                                                1                                      V                                                                  25                        ∘                                            ⁢                                                                                          ⁢                                              C                        .                                                                                            ·                                                                            V                                                                        35                          ∘                                                ⁢                                                                                                  ⁢                                                  C                          .                                                                                      -                                          V                                                                        15                          ∘                                                ⁢                                                                                                  ⁢                                                  C                          .                                                                                                                          20                    ⁢                                                                  (                        ∘                                            ⁢                                                                                          ⁢                                              C                        .                                            )                                                                                                                                Equation            ⁢                                                  ⁢                          (              1              )                                          
In Equation (1), αs represents a coefficient of linear expansion of a LiNbO3 substrate in the propagation direction of boundary wave. Table 1 shows characteristics of boundary acoustic waves that propagate through the configuration described above. ΔF in Table 1 is the change in frequency calculated from acoustic wave velocity Vs1 when the duty is changed by +0.01.
TABLE 1ItemPropagation propertiesType of boundary waveSH boundary wave mainly composed of SHcomponentAcoustic wave velocity3221 m/sVslTCD42.1 ppm/° C.K216.0%κ12/k00.15ΔF−2499 ppm
As shown in Table 1, the existing boundary acoustic wave device has a very large κ12/k0 of 0.15. This may result in a very large change in frequency ΔF of −2499 ppm.
Table 1 also shows that the boundary acoustic wave device has a large temperature coefficient of group delay time TCD of 42.1 ppm/° C.
In summary, a boundary acoustic wave device in which an IDT electrode made of Au is used and the energy of the boundary acoustic wave is confined at the interface between the first medium and the second medium exhibits a deteriorated temperature coefficient of group delay time TCD.